The Study of Australian Coal Maturity:  Relationship between Solid-State NMR Aromaticities and Organic Free-Radical Count

Samples of a range of Australian bituminous coals from Pelican-5, which is a petroleum drill site in the Bass Basin, located in Australia offshore between Victoria and Tasmania, were studied by 13 C solid-state nuclear magnetic resonance ( 13 C NMR) spectroscopy and electron spin resonance (ESR) spectroscopy. As expected, the fraction of carbon that is aromatic in these coals, measured using a single-pulse (Bloch decay) method, was higher than values obtained by the cross-polarization (CP) method, because some carbon observed using the single-pulse method is not observed by the CP method. Loss of signal through rapid spin−spin relaxation due to inorganic paramagnetics has been excluded as a source of differences in the aromaticity measurements; however, the organic free-electron content correlates well with observed spin−lattice relaxation times in the rotating frame and the difference between aromaticities measured by the Bloch decay and CP methods decreases as the electron count and rank increase. Some aromatic carbon is observed in Bloch decay experiments in low-rank coals but not by CP, because organic free electrons alter quantitation in CH/ T 1 ρ H dynamics more than they do in spin−spin relaxation. Because the aliphatic chains are longer at lower rank, they are more remote from the free electrons on aromatic rings and less influenced by free radicals on the aromatic rings.